Mass transfer characteristics of gas diffusion layer (GDL) are closely related to performance of polymer electrolyte fuel cells. Therefore, it is necessary to clarify the characteristics of water distribution relating to the microscopic conformation and oxygen diffusivity of GDL. A hybrid type carbon paper GDL with planar-distributed wettability is investigated for control of liquid water movement and distribution due to hydrophobic to hydrophilic areas that provide wettability differences in GDL and to achieve enhancement of both oxygen diffusion and moisture retention. Hybrid GDLs with different PTFE content were fabricated in an attempt to improve the oxygen diffusion characteristics. The effects of different PTFE contents on the oxygen diffusivity and water distribution were simultaneously measured and observed using galvanic cell oxygen absorber and X-ray radiography. The PTFE distribution was observed using scanning electron microscopy. The formation of oxygen diffusion paths was confirmed by X-ray radiography, where voids in the hybrid GDL were first formed in the hydrophobic regions and then spread to the untreated wetting region. Thus, the formation of oxygen diffusion paths enhanced the oxygen diffusion. In addition, the effects of local PTFE content in the hydrophobic region and the optimal amount of PTFE for hybrid GDL were elucidated.
The performance of polymer electrolyte fuel cells is influenced by moisture control in their gas diffusion layer (GDL). Therefore, to achieve suitable control, it is necessary to clarify the mass transfer characteristics within a GDL by high precision measurement of oxygen diffusivity. We have previously proposed that measurement of the effective oxygen diffusivity in a GDL containing moisture can be achieved using a galvanic cell oxygen absorber and demonstrated this to be an effective technique for the measurement of microporous media. However, the diffusion resistance of a single dry GDL is low, so that the margin of error in the oxygen diffusivity measurement is high. In this study, high precision measurement of the oxygen diffusivity in a GDL was developed further by analysis of the major error factors and modification of the measurement apparatus configuration. The results indicate a reduction in the maximum measurement error from 50 to 20% for a dry GDL with minimal diffusion resistance.
MicrogroovesWater management Flooding Gas channel a b s t r a c t Further performance improvement in polymer electrolyte fuel cell (PEFC) and their popularization face several problems even though they have been commercialized. The water behavior in PEFCs is closely related to cell performance. To enhance the cell performance, it is necessary to effectively remove the generated water. In this study, to improve the water control in gas channels, a novel gas channel with microgrooves, which are fabricated inside the channel walls, is applied. The generated water is removed through the microgrooves to facing side of the gas channel by capillary and shear forces from air flow. The performance of the PEFC with and without microgrooves was examined in various experimental conditions: cell temperature, relative humidity of gas, and air velocity. It was shown that the PEFC with microgrooves showed better performance than conventional PEFC without microgrooves. In particular, value of current density increased by approximately 16% when the air velocity was 8.0 m/s.
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